1. Field of the Invention
The present invention relates to siloxane polymer compositions. In particular, the invention relates to siloxane polymer compositions which have suitable properties for use in negative tone lithographic fabrication processes. The invention also relates to synthesis, polymerization and cross-linking of such compositions.
2. Description of Related Art
Photolithography is a common technique used in fabrication of semiconductor devices, such as integrated circuits (ICs), in flat panel display devices, such as liquid crystal displays, plasma displays, and organic light emitting displays, and in opto-electronic and photonic devices, such as waveguide and light-guide structures, gratings and photonic crystals.
In a photolithographic process, a layer of a photosensitive material is deposited on a substrate to form a coating. The deposited material layer is selectively exposed to some form of radiation, such as ultraviolet-light. An exposure tool and a mask or, in step-and-repeat projection-systems, a reticle are used to produce the desired selective exposure. The mask contains clear and opaque features that define the pattern to be created in the photosensitive material layer. The areas that are exposed to the light are made either soluble or insoluble by the use of a specific solvent known as a developer.
In the case where the irradiated (exposed) regions are soluble, a positive-image of the mask is produced in the photosensitive material. Such a material is called a positive tone photosensitive material [see attached FIG. 1(a)].
If, on the other hand, the non-irradiated regions are dissolved by the developer, a negative-image results. The material in this case is called a negative tone photosensitive material [see FIG. 1(b)].
Following the exposure the photosensitive material film must undergo a development step to turn the latent-image in the photosensitive material to the final image. In processes where the photosensitive material functions as a “sacrificial layer/structure”, the areas of the photosensitive material that remain after development serve to mask the substrate regions which they cover in subsequent etching or ion-implantation steps.
Locations from which the photosensitive material has been removed can be subjected to a variety of subtractive or additive processes that transfer the pattern onto the substrate surface. In processes where the photosensitive material functions as an “active layer/structure”, the areas of the photosensitive material that remain after development are used as they are in the final device/component structure and no additional etching or other subtractive or additive processes are needed.
Typically the positive tone photosensitive material developers are aqueous alkaline solutions, i.e. alkaline solutions diluted with water. To mention some typical examples: aqueous solutions of tetra methyl ammonium hydroxide (TMAH-water solutions) and potassium hydroxide (KOH-water solutions) are extensively used. These types of aqueous developers are favourable since they are commonly used by the industry and are also environmentally safe.
By contrast, the negative tone photosensitive material developers are typically organic solvent-based or -borne developers [e.g. acetone, isopropyl alcohol (IPA), methyl isobutyl ketone (MIBK), xylene and toluene] and this creates potentially severe environmental, health and safety (EHS) problems. Organic solvent-based developers are commonly used in lithographic processing of organo-siloxane polymer compositions. Typical developers that are used are acetone, IPA and MIBK. Water-soluble developers for negative tone siloxane materials have not been available.
Another problem relates to the fact that for many siloxane compositions resolution performance during photolithography is poor—for a 1.5 micrometer film, it is difficult to obtain a resolution better than 4-5 μm. The shape of the resulting structure from the photolithography step is also easily rounded.